When the Vescy family became extinct, Alnwick Castle and the surrounding manor were bequeathed to Antony Bek the Bishop of Durham.[3][9] The Percy family benefited from England's wars with Scotland; through his military accomplishments Henry Percy, 1st Baron Percy (1273–1314), enhanced his family's status in northern England. In 1309 he purchased the barony of Alnwick[10] from Bek, and it has been owned by the Percy family, the Earls and later Dukes of Northumberland since.[11] The stone castle Henry Percy bought was a modest affair, but he immediately began rebuilding. Though he did not live to see its completion, the building programme turned Alnwick into a major fortress along the Anglo-Scottish border. His son, also called Henry (1299–1352), continued the building.[12] The Abbot's Tower, the Middle Gateway and the Constable's Tower survive from this period.[11] The work at Alnwick Castle balanced military requirements with the family's residential needs. It set the template for castle renovations in the 14th century in northern England; several palace-fortresses, considered "extensive, opulent [and] theatrical" date from this period in the region, such as the castles of Bamburgh and Raby.[13] In 1345 the Percys acquired Warkworth Castle, also in Northumberland. Though Alnwick was considered more prestigious, Warkworth became the family's preferred residence.[14]

During the Wars of the Roses, castles were infrequently engaged in battle and conflict was generally based around combat in the field. Alnwick was one of three castles held by Lancastrian forces in 1461 and 1462, and it was there that the "only practical defence of a private castle" was made according to military historian D. J. Cathcart King.[16] It was held against King Edward until its surrender in mid-September 1461 after the Battle of Towton. Re-captured by Sir William Tailboys, during the winter it was surrendered by him to Hastings, Sir John Howard and Sir Ralph Grey of Heton in late July 1462. Grey was appointed captain but surrendered after a sharp siege in the early autumn. King Edward responded with vigour and when the Earl of Warwick arrived in November Queen Margaret and her French advisor, Pierre de Brézé were forced to sail to Scotland for help. They organised a mainly Scots relief force which, under George Douglas, 4th Earl of Angus and de Brézé, set out on 22 November. Warwick's army, commanded by the experienced Earl of Kent and the recently pardoned Lord Scales, prevented news getting through to the starving garrisons. As a result the nearby Bamburgh and Dunstanburgh castles soon agreed terms and surrendered. But Hungerford and Whittingham held Alnwick until Warwick was forced to withdraw when de Breze and Angus arrived on 5 January 1463.

The Lancastrians missed a chance to bring Warwick to battle instead being content to retire, leaving behind only a token force which surrendered next day.

By May 1463 Alnwick was in Lancastrian hands for the third time since Towton, betrayed by Grey of Heton who tricked the commander, Sir John Astley. Astley was imprisoned and Hungerford resumed command.

After Montagu's triumphs at Hedgeley Moor and Hexham in 1464 Warwick arrived before Alnwick on 23 June and received its surrender next day.

However in the 19th century Algernon, 4th Duke of Northumberland replaced much of Adam's architecture. Instead he paid Anthony Salvin £250,000 between 1854 and 1865 to remove the Gothic additions and other architectural work. Salvin is mostly responsible for the kitchen, the Prudhoe Tower, the palatial accommodation, and the layout of the inner ward.[17] According to the official website a large amount of Adam's work survives, but little or none of it remains in the principal rooms shown to the public, which were redecorated in an opulent Italianate style in the Victorian era by Luigi Canina.

The current duke and his family live in the castle, but occupy only a part of it. The castle is open to the public throughout the summer. After Windsor Castle, it is the second largest inhabited castle in England.[9][18] Alnwick was the tenth most-visited stately home in England according to the Historic Houses Association, with 195,504 visitors in 2006.[19]

Since World War II, parts of the castle have been used by various educational establishments: Firstly, by the Newcastle upon Tyne Church High School; then, from 1945 to 1977, as Alnwick College of Education, a teacher training college; and, since 1981, by St. Cloud State University of Minnesota as a branch campus forming part of their International Study Programme.

The castle is used as a stand-in for the exterior and interior of Hogwarts in the Harry Potter films, which has led to an increase in public interest.[9] Its appearance in the films has helped shape the public imagination regarding what castles should look like. Its condition contrasts with the vast majority of castles in the country, which are ruinous and unfit for habitation.[20] The wide angle images are computer generated. It has previously been a location used in Becket, The Black Adder, Robin Hood: Prince of Thieves and many others.

The castle grounds are also regularly in use as a cross country running venue as part of the North East Harrier League.

The River Aln flows past the north side of the Castle. There is a deep ravine to the south and east, separating the castle from the town.[21] By the 12th century, Alnwick Castle had assumed the general layout which it retains today. It is distinguished as one of the earliest castles in England to be built without a square keep.[9] The castle consists of two main rings of buildings. The inner ring is set around a small courtyard and contains the principal rooms. This structure is at the centre of a large bailey. As the central block was not large enough to contain all the accommodations required in later centuries, a large range of buildings was constructed along the south wall of the bailey. These two main areas of accommodation are connected by a link building. There are towers at regular intervals along the walls of the outer bailey. About a sixth of the bailey wall has been reduced almost to ground level on the bailey side to open up views into the park. Stable and service yards adjoin the castle outside the bailey; these would not have existed when the castle still had a military function.

Alnwick Castle has two parks. Immediately to the north of the castle is a relatively small park straddling the River Aln which was landscaped by Lancelot Brown ("Capability Brown") and Thomas Call in the 18th century; it is known locally as The Pastures. Nearby is the much larger Hulne Park, which contains the remains of Hulne Priory.

The castle is in good repair and used for many purposes. It provides a home for the present Duke and family and offices for Northumberland Estates, which manages the Duke's extensive farming and property holdings.

Alnwick's battlements are surmounted by carved figures dating from around 1300; historian Matthew Johnson notes that around this time there were several castles in northern England similarly decorated, such as Bothal, Lumley, and Raby.[22]

Adjacent to the castle, Jane Percy, Duchess of Northumberland, has initiated the establishment of The Alnwick Garden, a formal garden set around a cascading fountain. It cost £42 million (press release of 7 August 2003). The garden belongs to a charitable trust which is separate from the Northumberland Estates, but the Duke of Northumberland donated the 42-acre (17 ha) site and £9 million. The garden is designed by Jacques Wirtz and Peter Wirtz of Wirtz International based in Schoten, Belgium. The first phase of development, opened in October 2001, involved the creation of the fountain and initial planting of the gardens. In 2004 a large 6,000 sq ft (560 m2) 'tree house' complex, including a cafe, was opened. It is deemed one of the largest tree houses in the world.

Cathcart King, David James (1983), Castellarium Anglicanum: An Index and Bibliography of the Castles in England, Wales and the Islands. Volume II: Norfolk–Yorkshire and the Islands, London: Kraus International Publications, ISBN0-527-50110-7

Cathcart King, David James (1988), The Castle in England and Wales: an Interpretative History, London: Croom Helm, ISBN0-918400-08-2